The present invention relates to a vertical-cavity surface-emitting laser and, in particular, to a surface-emitting laser that achieves a high-power laser beam having a narrow angle of radiation by emitting phase-synchronized laser beams from a plurality of light-emitting portions, together with a method of fabrication thereof.
A topic of technical concern with surface-emitting lasers is how to emit a high-power laser beam that has a narrow angle of radiation. Japanese Patent Application Laid-Open No. 8-340156 discloses a surface-emitting laser that emits phase-synchronized laser beams from a plurality of light-emitting portions, to produce what appears to be a single laser beam, due to the interference effect of light. The apparently single laser beam was observed to have a high power level and a narrow angle of radiation.
In this surface-emitting laser, a columnar portion is formed on a cladding layer on a light-emitting side, an embedding layer is formed around the columnar portion, and an electrode on a light-emitting side has a plurality of aperture portions, where peripheral portions of the aperture portions are in contact with the columnar portion. The lasing mode of light directly under the aperture portions of the electrode is different from that directly under the peripheral portions thereof, but the plurality of laser beams emitted from the aperture portions appears to form a single laser beam.
An insulating material is used as the embedding layer in this surface-emitting laser. The refractive index of an oxide such as SiO2, a nitride such as Si3N4, or a compound from Group II-VI such as ZnSe that is generally used as the insulating embedding layer is greatly different from the refractive index of a compound from Group III-V such as GaAs that is a compositional element of the cladding layer. Since this means that light is enclosed excessively within the central portion of the resonator, it is impossible to cut out the higher transverse modes.
The present invention was devised in order to solve the above problems of the prior art and thus provides a surface-emitting laser wherein the transverse modes are controlled and phase-synchronized laser beams are emitted from a plurality of light-emitting portions to produce what appears to be a single laser beam, and a method of fabrication thereof.
(1) A vertical-cavity surface-emitting laser in accordance with an aspect of the present invention comprises:
a columnar portion formed of at least part of a reflective mirror on a light-emitting side; an embedding layer surrounding the periphery of the columnar portion; an upper electrode formed on the columnar portion and the embedding layer, and an insulating layer formed below the columnar portion and the embedding layer, wherein:
a plurality of electrode aperture portions are formed in the upper electrode above the columnar portion;
a plurality of insulation aperture portions are formed in the insulation layer at positions corresponding to the electrode aperture portions; and
the embedding layer has an absolute refractive index that is slightly smaller than that of the columnar portion.
With this aspect of the invention, insulation aperture portions are formed in the insulation layer so that current is supplied from the upper electrode, through the insulation aperture portions, and into the active layer. Light generated in correspondence with the insulation aperture portions has the same lasing mode. A plurality of phase-synchronized laser beams is thus emitted from the plurality of electrode aperture portions. This plurality of laser beams forms a single high-power laser beam with a narrow angle of radiation.
This aspect of the invention also has an embedding layer surrounding the periphery of the columnar portion, and the absolute refractive index (refractive index with respect to vacuum) of the embedding layer is made to be smaller than the absolute refractive index of the columnar portion. This configuration makes it possible to ensure that all the light is totally reflected within the columnar portion and is enclosed thereby, in a manner similar to an optical fiber.
Note that it is known that, when there is a large difference in absolute refractive indices between the core and cladding of an optical fiber, a large number of modes can be transmitted thereby. These transverse modes can therefore be controlled by ensuring that the difference in absolute refractive indices is small.
In a similar manner, the difference in absolute refractive indices is small in this aspect of the invention, so transverse modes can be controlled.
(2) With respect to the above described surface-emitting laser, it is preferable that:
a material forming the columnar portion is made to be single crystal; and
a material forming the embedding layer is the same material as that of the columnar portion, but is made to be non-single crystal.
The same material has a higher density and a higher absolute refractive index when it is made single crystalline, whereas it has a slightly lower density and a slightly lower absolute refractive index when it is made non-single crystalline (polycrystalline or non-crystalline). It is therefore possible to change the absolute refractive index slightly by making the material either single crystalline or non-single crystalline.
(3) With respect to the above described surface-emitting laser, it is preferable that:
each of the electrode aperture portions has a diameter on the order of 1 to 6 xcexcm, and the distance between adjacent electrode aperture portions is approximately not more than 7 xcexcm.
(4) With respect to the above described surface-emitting laser, it is preferable that:
the embedding layer has a low electrical resistance.
Lowering the electrical resistance of the embedding layer in this manner makes it possible to restrain the generation of heat therein.
(5) A method of fabricating a vertical-cavity surface-emitting laser in accordance with another aspect of the present invention comprises the steps of:
forming a single crystal layer at a position above an active layer but below a reflective mirror on a light-emitting side;
forming a non-single crystal insulation layer on the single crystal layer;
forming a plurality of insulation aperture portions in the insulation layer, to form exposed portions of the single crystal layer;
growing a multi-layer film non-selectively on the insulation layer that comprises the insulation aperture portions; and
forming an upper electrode on the multi-layer film, the upper electrode having electrode aperture portions corresponding to the insulation aperture portions,
wherein the multi-layer film is made to be non-single crystal on the non-single crystal insulation layer and single crystal above the insulation aperture portions.
With this aspect of the invention, the single crystal layer is exposed from the insulation aperture portions, so that the multi-layer film that is grown by non-selective growth becomes a single crystal columnar portion on these insulation aperture portions and a non-single crystal (polycrystalline or non-crystalline) embedding layer on the periphery thereof. Thus the above described surface-emitting laser can be constructed in a simple manner.
(6) With respect to the above described method of fabricating a surface-emitting laser, it is preferable that:
the multi-layer film has a low electrical resistance.
Lowering the electrical resistance of the multi-layer film in this manner makes it possible to control the heat generated within the embedding layer.